1. Field of the Invention
The present invention relates to a torque sensor which detects a torque on the basis of an inductance change of a pair of coils, and in particular, to a torque sensor which can detect an abnormal state such as a connective fault of a coil.
2. Description of the Related Art
Relating to this type of torque sensor detecting a connective fault of a coil, there is a torque sensor disclosed in Japanese Patent Application Laid-Open No. 8-136366.
The torque sensor disclosed in the above publication includes a pair of coils whose inductance mutually changes in a reverse direction in accordance with a torque, and a differential amplifier circuit which differentially amplifies a pair of detection voltages induced to the pair of coils when receiving the detection voltages, and thus, detects a torque from an output of the differential amplifier circuit.
Then, when detecting an abnormal state from the detection voltage induced to the pair of coils, the torque sensor controls the differential amplifier circuit so that an output of the differential amplifier circuit becomes a predetermined value other than a steady-state output range.
When the output of the differential amplifier circuit becomes the above predetermined value, the torque sensor makes a decision that an abnormal state such as a connective fault of a coil takes place.
For this reason, the following comparator means must be independently provided; more specifically, the comparator means makes a decision whether or not the output of the differential amplifier circuit is other than a steady-state output range.
The present invention has been achieved with a view of the aforesaid problem in the prior art. It is, therefore, an object of the present invention to provide a torque sensor which can detect an abnormal state such as a connective fault of coil from an output of a differential amplifier circuit without newly providing a comparator means independently from the torque sensor.
To achieve the above object, the present invention provides a torque sensor comprising: a pair of coils having inductances changing mutually in a reverse direction in accordance with a torque; a first differential amplifier means inputting first and second sub-voltages based on each inductance change of the both coils, and amplifying a difference between the first and second sub-voltages so as to output it as a first main voltage; a second differential amplifier means inputting third and fourth sub-voltages based on each inductance change of the both coils, and amplifying a difference between the third and fourth sub-voltages so as to output it as a second main voltage; and an abnormal state signal outputting means outputting an abnormal state signal when a difference between the first and second main voltages exceeds a predetermined allowable range, the torque sensor further including: an abnormal state comparator means for judging an abnormal state of the coils from the first, second, third and fourth sub-voltages; and a voltage displacement means for displacing the second main voltage so as to exceed the predetermined allowable range when the abnormal state comparator means makes a decision that an abnormal state occurs, the abnormal state signal outputting means outputting the abnormal state signal when an abnormal state occurs in the coils.